Natural rubber has hitherto been used widely as industrial products, such as automobile tires, belts and pressure-sensitive adhesives, and household goods, such as gloves. These natural rubber articles are generally produced by coagulating the rubber content of a natural rubber latex to obtain raw rubber called crepe rubber or smoked sheet rubber, and further processing the raw rubber through steps of mastication, compounding of additives, molding, and vulcanization.
It was recently reported that medical tools made of natural rubber, such as surgical gloves, various catheters, and analgesic masks, provoke labored respiration or anaphylactoid symptoms, such as vascular edema, nettle rash, detelectasis and cyanosis, in patients. Cases were also reported in which women with a history of allergy suffered a pain in the hands, nettle rash or vascular edema around the eyes when they used rubber gloves made of natural rubber.
These symptoms seem to be attributed to the protein present in natural rubber. Food and Drug Administration (FDA), U.S.A. has called on manufactures of natural rubber to reduce the protein content. It has therefore been demanded to remove protein from natural rubber.
Natural rubber is obtained from Hevea trees as a latex containing a rubber content, water, protein, inorganic salts, and other impurities. The latex oozing out from the tapped trunk of a rubber plant is collected in a cup, gathered at a refining factory where it is coagulated to obtain raw rubber (crepe rubber or smoked sheet rubber) or concentrated by centrifugation to obtain a purified latex.
The protein content in natural rubber has usually been expressed in terms of a nitrogen content (N %) determined by a Kjeldahl method multiplied by 6.3. The present inventors discovered that the proteins in raw rubber obtained from a latex can be confirmed by infrared absorption at 3280 cm.sup.-1 characteristic of polypeptide.
The present inventors previously found that a deproteinized natural rubber latex showing no IR absorption at 3280 cm.sup.-1 can be obtained by a process comprising treating a natural rubber latex with a protease and a surfactant either simultaneously or successively and, after allowing the system to stand for a given period of time, recovering the rubber particles by centrifugation (see Japanese Patent Application Nos. 208754 to 208758/92 (corresponding to EP-A-0 584 597)).
As a method for recovering the rubber particles from natural rubber latex, a method comprising adding an acid (e.g., formic acid and acetic acid) to a latex and a method comprising adding an inorganic salt (e.g., calcium chloride, aluminum sulfate and calcium nitrate) are generally known.
For example, a latex is diluted to have a solids content of about 15 to 20% by weight, and formic acid is subsequently added thereto in a concentration of from 0.1 to 1% by weight to agglomerate the rubber particles, which is then separated, washed and dried to recover it.
As compared with the general latex on the market, the above-mentioned deproteinized latex is very poor in the mechanical stability. However, in contrast, when it is attempted to recover the rubber content in the deproteinized latex by addition of an acid, the deproteinized latex causes only insufficient agglomeration to recover the rubber content. On the other hand, when the above-mentioned method wherein an inorganic salt is added to undergo agglomeration is employed, metal ions are unavoidably incorporated into the resulting solid rubber, which causes problems such as reductions in physical properties due to moisture absorption, blooming, retardation of vulcanization, and a reduction in resistance to deterioration on aging.